xref: /linux/fs/mpage.c (revision be3c215342956313845308e0e631341e62370a2b)
1  // SPDX-License-Identifier: GPL-2.0
2  /*
3   * fs/mpage.c
4   *
5   * Copyright (C) 2002, Linus Torvalds.
6   *
7   * Contains functions related to preparing and submitting BIOs which contain
8   * multiple pagecache pages.
9   *
10   * 15May2002	Andrew Morton
11   *		Initial version
12   * 27Jun2002	axboe@suse.de
13   *		use bio_add_page() to build bio's just the right size
14   */
15  
16  #include <linux/kernel.h>
17  #include <linux/export.h>
18  #include <linux/mm.h>
19  #include <linux/kdev_t.h>
20  #include <linux/gfp.h>
21  #include <linux/bio.h>
22  #include <linux/fs.h>
23  #include <linux/buffer_head.h>
24  #include <linux/blkdev.h>
25  #include <linux/highmem.h>
26  #include <linux/prefetch.h>
27  #include <linux/mpage.h>
28  #include <linux/mm_inline.h>
29  #include <linux/writeback.h>
30  #include <linux/backing-dev.h>
31  #include <linux/pagevec.h>
32  #include "internal.h"
33  
34  /*
35   * I/O completion handler for multipage BIOs.
36   *
37   * The mpage code never puts partial pages into a BIO (except for end-of-file).
38   * If a page does not map to a contiguous run of blocks then it simply falls
39   * back to block_read_full_folio().
40   *
41   * Why is this?  If a page's completion depends on a number of different BIOs
42   * which can complete in any order (or at the same time) then determining the
43   * status of that page is hard.  See end_buffer_async_read() for the details.
44   * There is no point in duplicating all that complexity.
45   */
46  static void mpage_read_end_io(struct bio *bio)
47  {
48  	struct folio_iter fi;
49  	int err = blk_status_to_errno(bio->bi_status);
50  
51  	bio_for_each_folio_all(fi, bio) {
52  		if (err)
53  			folio_set_error(fi.folio);
54  		else
55  			folio_mark_uptodate(fi.folio);
56  		folio_unlock(fi.folio);
57  	}
58  
59  	bio_put(bio);
60  }
61  
62  static void mpage_write_end_io(struct bio *bio)
63  {
64  	struct folio_iter fi;
65  	int err = blk_status_to_errno(bio->bi_status);
66  
67  	bio_for_each_folio_all(fi, bio) {
68  		if (err) {
69  			folio_set_error(fi.folio);
70  			mapping_set_error(fi.folio->mapping, err);
71  		}
72  		folio_end_writeback(fi.folio);
73  	}
74  
75  	bio_put(bio);
76  }
77  
78  static struct bio *mpage_bio_submit_read(struct bio *bio)
79  {
80  	bio->bi_end_io = mpage_read_end_io;
81  	guard_bio_eod(bio);
82  	submit_bio(bio);
83  	return NULL;
84  }
85  
86  static struct bio *mpage_bio_submit_write(struct bio *bio)
87  {
88  	bio->bi_end_io = mpage_write_end_io;
89  	guard_bio_eod(bio);
90  	submit_bio(bio);
91  	return NULL;
92  }
93  
94  /*
95   * support function for mpage_readahead.  The fs supplied get_block might
96   * return an up to date buffer.  This is used to map that buffer into
97   * the page, which allows read_folio to avoid triggering a duplicate call
98   * to get_block.
99   *
100   * The idea is to avoid adding buffers to pages that don't already have
101   * them.  So when the buffer is up to date and the page size == block size,
102   * this marks the page up to date instead of adding new buffers.
103   */
104  static void map_buffer_to_folio(struct folio *folio, struct buffer_head *bh,
105  		int page_block)
106  {
107  	struct inode *inode = folio->mapping->host;
108  	struct buffer_head *page_bh, *head;
109  	int block = 0;
110  
111  	head = folio_buffers(folio);
112  	if (!head) {
113  		/*
114  		 * don't make any buffers if there is only one buffer on
115  		 * the folio and the folio just needs to be set up to date
116  		 */
117  		if (inode->i_blkbits == PAGE_SHIFT &&
118  		    buffer_uptodate(bh)) {
119  			folio_mark_uptodate(folio);
120  			return;
121  		}
122  		create_empty_buffers(&folio->page, i_blocksize(inode), 0);
123  		head = folio_buffers(folio);
124  	}
125  
126  	page_bh = head;
127  	do {
128  		if (block == page_block) {
129  			page_bh->b_state = bh->b_state;
130  			page_bh->b_bdev = bh->b_bdev;
131  			page_bh->b_blocknr = bh->b_blocknr;
132  			break;
133  		}
134  		page_bh = page_bh->b_this_page;
135  		block++;
136  	} while (page_bh != head);
137  }
138  
139  struct mpage_readpage_args {
140  	struct bio *bio;
141  	struct folio *folio;
142  	unsigned int nr_pages;
143  	bool is_readahead;
144  	sector_t last_block_in_bio;
145  	struct buffer_head map_bh;
146  	unsigned long first_logical_block;
147  	get_block_t *get_block;
148  };
149  
150  /*
151   * This is the worker routine which does all the work of mapping the disk
152   * blocks and constructs largest possible bios, submits them for IO if the
153   * blocks are not contiguous on the disk.
154   *
155   * We pass a buffer_head back and forth and use its buffer_mapped() flag to
156   * represent the validity of its disk mapping and to decide when to do the next
157   * get_block() call.
158   */
159  static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
160  {
161  	struct folio *folio = args->folio;
162  	struct inode *inode = folio->mapping->host;
163  	const unsigned blkbits = inode->i_blkbits;
164  	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
165  	const unsigned blocksize = 1 << blkbits;
166  	struct buffer_head *map_bh = &args->map_bh;
167  	sector_t block_in_file;
168  	sector_t last_block;
169  	sector_t last_block_in_file;
170  	sector_t blocks[MAX_BUF_PER_PAGE];
171  	unsigned page_block;
172  	unsigned first_hole = blocks_per_page;
173  	struct block_device *bdev = NULL;
174  	int length;
175  	int fully_mapped = 1;
176  	blk_opf_t opf = REQ_OP_READ;
177  	unsigned nblocks;
178  	unsigned relative_block;
179  	gfp_t gfp = mapping_gfp_constraint(folio->mapping, GFP_KERNEL);
180  
181  	/* MAX_BUF_PER_PAGE, for example */
182  	VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
183  
184  	if (args->is_readahead) {
185  		opf |= REQ_RAHEAD;
186  		gfp |= __GFP_NORETRY | __GFP_NOWARN;
187  	}
188  
189  	if (folio_buffers(folio))
190  		goto confused;
191  
192  	block_in_file = (sector_t)folio->index << (PAGE_SHIFT - blkbits);
193  	last_block = block_in_file + args->nr_pages * blocks_per_page;
194  	last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
195  	if (last_block > last_block_in_file)
196  		last_block = last_block_in_file;
197  	page_block = 0;
198  
199  	/*
200  	 * Map blocks using the result from the previous get_blocks call first.
201  	 */
202  	nblocks = map_bh->b_size >> blkbits;
203  	if (buffer_mapped(map_bh) &&
204  			block_in_file > args->first_logical_block &&
205  			block_in_file < (args->first_logical_block + nblocks)) {
206  		unsigned map_offset = block_in_file - args->first_logical_block;
207  		unsigned last = nblocks - map_offset;
208  
209  		for (relative_block = 0; ; relative_block++) {
210  			if (relative_block == last) {
211  				clear_buffer_mapped(map_bh);
212  				break;
213  			}
214  			if (page_block == blocks_per_page)
215  				break;
216  			blocks[page_block] = map_bh->b_blocknr + map_offset +
217  						relative_block;
218  			page_block++;
219  			block_in_file++;
220  		}
221  		bdev = map_bh->b_bdev;
222  	}
223  
224  	/*
225  	 * Then do more get_blocks calls until we are done with this folio.
226  	 */
227  	map_bh->b_folio = folio;
228  	while (page_block < blocks_per_page) {
229  		map_bh->b_state = 0;
230  		map_bh->b_size = 0;
231  
232  		if (block_in_file < last_block) {
233  			map_bh->b_size = (last_block-block_in_file) << blkbits;
234  			if (args->get_block(inode, block_in_file, map_bh, 0))
235  				goto confused;
236  			args->first_logical_block = block_in_file;
237  		}
238  
239  		if (!buffer_mapped(map_bh)) {
240  			fully_mapped = 0;
241  			if (first_hole == blocks_per_page)
242  				first_hole = page_block;
243  			page_block++;
244  			block_in_file++;
245  			continue;
246  		}
247  
248  		/* some filesystems will copy data into the page during
249  		 * the get_block call, in which case we don't want to
250  		 * read it again.  map_buffer_to_folio copies the data
251  		 * we just collected from get_block into the folio's buffers
252  		 * so read_folio doesn't have to repeat the get_block call
253  		 */
254  		if (buffer_uptodate(map_bh)) {
255  			map_buffer_to_folio(folio, map_bh, page_block);
256  			goto confused;
257  		}
258  
259  		if (first_hole != blocks_per_page)
260  			goto confused;		/* hole -> non-hole */
261  
262  		/* Contiguous blocks? */
263  		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
264  			goto confused;
265  		nblocks = map_bh->b_size >> blkbits;
266  		for (relative_block = 0; ; relative_block++) {
267  			if (relative_block == nblocks) {
268  				clear_buffer_mapped(map_bh);
269  				break;
270  			} else if (page_block == blocks_per_page)
271  				break;
272  			blocks[page_block] = map_bh->b_blocknr+relative_block;
273  			page_block++;
274  			block_in_file++;
275  		}
276  		bdev = map_bh->b_bdev;
277  	}
278  
279  	if (first_hole != blocks_per_page) {
280  		folio_zero_segment(folio, first_hole << blkbits, PAGE_SIZE);
281  		if (first_hole == 0) {
282  			folio_mark_uptodate(folio);
283  			folio_unlock(folio);
284  			goto out;
285  		}
286  	} else if (fully_mapped) {
287  		folio_set_mappedtodisk(folio);
288  	}
289  
290  	/*
291  	 * This folio will go to BIO.  Do we need to send this BIO off first?
292  	 */
293  	if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
294  		args->bio = mpage_bio_submit_read(args->bio);
295  
296  alloc_new:
297  	if (args->bio == NULL) {
298  		args->bio = bio_alloc(bdev, bio_max_segs(args->nr_pages), opf,
299  				      gfp);
300  		if (args->bio == NULL)
301  			goto confused;
302  		args->bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
303  	}
304  
305  	length = first_hole << blkbits;
306  	if (!bio_add_folio(args->bio, folio, length, 0)) {
307  		args->bio = mpage_bio_submit_read(args->bio);
308  		goto alloc_new;
309  	}
310  
311  	relative_block = block_in_file - args->first_logical_block;
312  	nblocks = map_bh->b_size >> blkbits;
313  	if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
314  	    (first_hole != blocks_per_page))
315  		args->bio = mpage_bio_submit_read(args->bio);
316  	else
317  		args->last_block_in_bio = blocks[blocks_per_page - 1];
318  out:
319  	return args->bio;
320  
321  confused:
322  	if (args->bio)
323  		args->bio = mpage_bio_submit_read(args->bio);
324  	if (!folio_test_uptodate(folio))
325  		block_read_full_folio(folio, args->get_block);
326  	else
327  		folio_unlock(folio);
328  	goto out;
329  }
330  
331  /**
332   * mpage_readahead - start reads against pages
333   * @rac: Describes which pages to read.
334   * @get_block: The filesystem's block mapper function.
335   *
336   * This function walks the pages and the blocks within each page, building and
337   * emitting large BIOs.
338   *
339   * If anything unusual happens, such as:
340   *
341   * - encountering a page which has buffers
342   * - encountering a page which has a non-hole after a hole
343   * - encountering a page with non-contiguous blocks
344   *
345   * then this code just gives up and calls the buffer_head-based read function.
346   * It does handle a page which has holes at the end - that is a common case:
347   * the end-of-file on blocksize < PAGE_SIZE setups.
348   *
349   * BH_Boundary explanation:
350   *
351   * There is a problem.  The mpage read code assembles several pages, gets all
352   * their disk mappings, and then submits them all.  That's fine, but obtaining
353   * the disk mappings may require I/O.  Reads of indirect blocks, for example.
354   *
355   * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
356   * submitted in the following order:
357   *
358   * 	12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
359   *
360   * because the indirect block has to be read to get the mappings of blocks
361   * 13,14,15,16.  Obviously, this impacts performance.
362   *
363   * So what we do it to allow the filesystem's get_block() function to set
364   * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
365   * after this one will require I/O against a block which is probably close to
366   * this one.  So you should push what I/O you have currently accumulated.
367   *
368   * This all causes the disk requests to be issued in the correct order.
369   */
370  void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
371  {
372  	struct folio *folio;
373  	struct mpage_readpage_args args = {
374  		.get_block = get_block,
375  		.is_readahead = true,
376  	};
377  
378  	while ((folio = readahead_folio(rac))) {
379  		prefetchw(&folio->flags);
380  		args.folio = folio;
381  		args.nr_pages = readahead_count(rac);
382  		args.bio = do_mpage_readpage(&args);
383  	}
384  	if (args.bio)
385  		mpage_bio_submit_read(args.bio);
386  }
387  EXPORT_SYMBOL(mpage_readahead);
388  
389  /*
390   * This isn't called much at all
391   */
392  int mpage_read_folio(struct folio *folio, get_block_t get_block)
393  {
394  	struct mpage_readpage_args args = {
395  		.folio = folio,
396  		.nr_pages = 1,
397  		.get_block = get_block,
398  	};
399  
400  	args.bio = do_mpage_readpage(&args);
401  	if (args.bio)
402  		mpage_bio_submit_read(args.bio);
403  	return 0;
404  }
405  EXPORT_SYMBOL(mpage_read_folio);
406  
407  /*
408   * Writing is not so simple.
409   *
410   * If the page has buffers then they will be used for obtaining the disk
411   * mapping.  We only support pages which are fully mapped-and-dirty, with a
412   * special case for pages which are unmapped at the end: end-of-file.
413   *
414   * If the page has no buffers (preferred) then the page is mapped here.
415   *
416   * If all blocks are found to be contiguous then the page can go into the
417   * BIO.  Otherwise fall back to the mapping's writepage().
418   *
419   * FIXME: This code wants an estimate of how many pages are still to be
420   * written, so it can intelligently allocate a suitably-sized BIO.  For now,
421   * just allocate full-size (16-page) BIOs.
422   */
423  
424  struct mpage_data {
425  	struct bio *bio;
426  	sector_t last_block_in_bio;
427  	get_block_t *get_block;
428  };
429  
430  /*
431   * We have our BIO, so we can now mark the buffers clean.  Make
432   * sure to only clean buffers which we know we'll be writing.
433   */
434  static void clean_buffers(struct page *page, unsigned first_unmapped)
435  {
436  	unsigned buffer_counter = 0;
437  	struct buffer_head *bh, *head;
438  	if (!page_has_buffers(page))
439  		return;
440  	head = page_buffers(page);
441  	bh = head;
442  
443  	do {
444  		if (buffer_counter++ == first_unmapped)
445  			break;
446  		clear_buffer_dirty(bh);
447  		bh = bh->b_this_page;
448  	} while (bh != head);
449  
450  	/*
451  	 * we cannot drop the bh if the page is not uptodate or a concurrent
452  	 * read_folio would fail to serialize with the bh and it would read from
453  	 * disk before we reach the platter.
454  	 */
455  	if (buffer_heads_over_limit && PageUptodate(page))
456  		try_to_free_buffers(page_folio(page));
457  }
458  
459  /*
460   * For situations where we want to clean all buffers attached to a page.
461   * We don't need to calculate how many buffers are attached to the page,
462   * we just need to specify a number larger than the maximum number of buffers.
463   */
464  void clean_page_buffers(struct page *page)
465  {
466  	clean_buffers(page, ~0U);
467  }
468  
469  static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
470  		      void *data)
471  {
472  	struct mpage_data *mpd = data;
473  	struct bio *bio = mpd->bio;
474  	struct address_space *mapping = folio->mapping;
475  	struct inode *inode = mapping->host;
476  	const unsigned blkbits = inode->i_blkbits;
477  	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
478  	sector_t last_block;
479  	sector_t block_in_file;
480  	sector_t blocks[MAX_BUF_PER_PAGE];
481  	unsigned page_block;
482  	unsigned first_unmapped = blocks_per_page;
483  	struct block_device *bdev = NULL;
484  	int boundary = 0;
485  	sector_t boundary_block = 0;
486  	struct block_device *boundary_bdev = NULL;
487  	size_t length;
488  	struct buffer_head map_bh;
489  	loff_t i_size = i_size_read(inode);
490  	int ret = 0;
491  	struct buffer_head *head = folio_buffers(folio);
492  
493  	if (head) {
494  		struct buffer_head *bh = head;
495  
496  		/* If they're all mapped and dirty, do it */
497  		page_block = 0;
498  		do {
499  			BUG_ON(buffer_locked(bh));
500  			if (!buffer_mapped(bh)) {
501  				/*
502  				 * unmapped dirty buffers are created by
503  				 * block_dirty_folio -> mmapped data
504  				 */
505  				if (buffer_dirty(bh))
506  					goto confused;
507  				if (first_unmapped == blocks_per_page)
508  					first_unmapped = page_block;
509  				continue;
510  			}
511  
512  			if (first_unmapped != blocks_per_page)
513  				goto confused;	/* hole -> non-hole */
514  
515  			if (!buffer_dirty(bh) || !buffer_uptodate(bh))
516  				goto confused;
517  			if (page_block) {
518  				if (bh->b_blocknr != blocks[page_block-1] + 1)
519  					goto confused;
520  			}
521  			blocks[page_block++] = bh->b_blocknr;
522  			boundary = buffer_boundary(bh);
523  			if (boundary) {
524  				boundary_block = bh->b_blocknr;
525  				boundary_bdev = bh->b_bdev;
526  			}
527  			bdev = bh->b_bdev;
528  		} while ((bh = bh->b_this_page) != head);
529  
530  		if (first_unmapped)
531  			goto page_is_mapped;
532  
533  		/*
534  		 * Page has buffers, but they are all unmapped. The page was
535  		 * created by pagein or read over a hole which was handled by
536  		 * block_read_full_folio().  If this address_space is also
537  		 * using mpage_readahead then this can rarely happen.
538  		 */
539  		goto confused;
540  	}
541  
542  	/*
543  	 * The page has no buffers: map it to disk
544  	 */
545  	BUG_ON(!folio_test_uptodate(folio));
546  	block_in_file = (sector_t)folio->index << (PAGE_SHIFT - blkbits);
547  	/*
548  	 * Whole page beyond EOF? Skip allocating blocks to avoid leaking
549  	 * space.
550  	 */
551  	if (block_in_file >= (i_size + (1 << blkbits) - 1) >> blkbits)
552  		goto page_is_mapped;
553  	last_block = (i_size - 1) >> blkbits;
554  	map_bh.b_folio = folio;
555  	for (page_block = 0; page_block < blocks_per_page; ) {
556  
557  		map_bh.b_state = 0;
558  		map_bh.b_size = 1 << blkbits;
559  		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
560  			goto confused;
561  		if (!buffer_mapped(&map_bh))
562  			goto confused;
563  		if (buffer_new(&map_bh))
564  			clean_bdev_bh_alias(&map_bh);
565  		if (buffer_boundary(&map_bh)) {
566  			boundary_block = map_bh.b_blocknr;
567  			boundary_bdev = map_bh.b_bdev;
568  		}
569  		if (page_block) {
570  			if (map_bh.b_blocknr != blocks[page_block-1] + 1)
571  				goto confused;
572  		}
573  		blocks[page_block++] = map_bh.b_blocknr;
574  		boundary = buffer_boundary(&map_bh);
575  		bdev = map_bh.b_bdev;
576  		if (block_in_file == last_block)
577  			break;
578  		block_in_file++;
579  	}
580  	BUG_ON(page_block == 0);
581  
582  	first_unmapped = page_block;
583  
584  page_is_mapped:
585  	/* Don't bother writing beyond EOF, truncate will discard the folio */
586  	if (folio_pos(folio) >= i_size)
587  		goto confused;
588  	length = folio_size(folio);
589  	if (folio_pos(folio) + length > i_size) {
590  		/*
591  		 * The page straddles i_size.  It must be zeroed out on each
592  		 * and every writepage invocation because it may be mmapped.
593  		 * "A file is mapped in multiples of the page size.  For a file
594  		 * that is not a multiple of the page size, the remaining memory
595  		 * is zeroed when mapped, and writes to that region are not
596  		 * written out to the file."
597  		 */
598  		length = i_size - folio_pos(folio);
599  		folio_zero_segment(folio, length, folio_size(folio));
600  	}
601  
602  	/*
603  	 * This page will go to BIO.  Do we need to send this BIO off first?
604  	 */
605  	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
606  		bio = mpage_bio_submit_write(bio);
607  
608  alloc_new:
609  	if (bio == NULL) {
610  		bio = bio_alloc(bdev, BIO_MAX_VECS,
611  				REQ_OP_WRITE | wbc_to_write_flags(wbc),
612  				GFP_NOFS);
613  		bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
614  		wbc_init_bio(wbc, bio);
615  	}
616  
617  	/*
618  	 * Must try to add the page before marking the buffer clean or
619  	 * the confused fail path above (OOM) will be very confused when
620  	 * it finds all bh marked clean (i.e. it will not write anything)
621  	 */
622  	wbc_account_cgroup_owner(wbc, &folio->page, folio_size(folio));
623  	length = first_unmapped << blkbits;
624  	if (!bio_add_folio(bio, folio, length, 0)) {
625  		bio = mpage_bio_submit_write(bio);
626  		goto alloc_new;
627  	}
628  
629  	clean_buffers(&folio->page, first_unmapped);
630  
631  	BUG_ON(folio_test_writeback(folio));
632  	folio_start_writeback(folio);
633  	folio_unlock(folio);
634  	if (boundary || (first_unmapped != blocks_per_page)) {
635  		bio = mpage_bio_submit_write(bio);
636  		if (boundary_block) {
637  			write_boundary_block(boundary_bdev,
638  					boundary_block, 1 << blkbits);
639  		}
640  	} else {
641  		mpd->last_block_in_bio = blocks[blocks_per_page - 1];
642  	}
643  	goto out;
644  
645  confused:
646  	if (bio)
647  		bio = mpage_bio_submit_write(bio);
648  
649  	/*
650  	 * The caller has a ref on the inode, so *mapping is stable
651  	 */
652  	ret = block_write_full_page(&folio->page, mpd->get_block, wbc);
653  	mapping_set_error(mapping, ret);
654  out:
655  	mpd->bio = bio;
656  	return ret;
657  }
658  
659  /**
660   * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
661   * @mapping: address space structure to write
662   * @wbc: subtract the number of written pages from *@wbc->nr_to_write
663   * @get_block: the filesystem's block mapper function.
664   *
665   * This is a library function, which implements the writepages()
666   * address_space_operation.
667   */
668  int
669  mpage_writepages(struct address_space *mapping,
670  		struct writeback_control *wbc, get_block_t get_block)
671  {
672  	struct mpage_data mpd = {
673  		.get_block	= get_block,
674  	};
675  	struct blk_plug plug;
676  	int ret;
677  
678  	blk_start_plug(&plug);
679  	ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
680  	if (mpd.bio)
681  		mpage_bio_submit_write(mpd.bio);
682  	blk_finish_plug(&plug);
683  	return ret;
684  }
685  EXPORT_SYMBOL(mpage_writepages);
686